Aluminum plating of surfaces



United States Patent O US. 'Cl. 11746 18 Claims ABSTRACT OF THEDISCLOSURE Where an object is to be plated with aluminum alkyldecomposition in liquid phase aluminum by heating said object to atemperature above the decomposition temperature of the aluminum alkyland thereafter contacting the heated article with a solution of aluminumalkyl, the article to be plated is first coated with a heavy hydrocarbonto protect the article from oxidation.

CROSS REFERENCE This is a continuation-in-part of my application havingSer. No. 444,392, filed Mar. 31, 1965, now abandoned.

FIELD OF INVENTION This invention relates to a method of coating asurface with aluminum. In one of its more specific aspects thisinvention relates to a method of protecting a surface against oxidationprior to contacting said surface With a plating batch. In another aspectthis invention relates to liquid bath plating with aluminum.

BACKGROUND In the earlier filed application, I disclosed coating thearticle to be treated with high boiling waxes and mineral oils.Subsequent to-filing the parent application, I have found that certaindiphenyl alkanes and certain dialkyl benzenes are especially suited ascoating materials. While I was not restricted to separate heating in theearlier application, I wish to specifically disclosed heating thearticle to be plated in the coating oil in this application.

Various systems have been used to coat an oxidizable surface such asmild steel with aluminum. Cladding techniques have been employed whereinan aluminum sheet is bonded to the desired substrate or surface byextensive surface treatment of the article followed by a combination ofheat and pressure. High operating pressures are necessary, thus limitingthe type of articles which could be covered. Also, serious corrosion andoperating problems are encountered.

The various well known vacuum techniques produce quite thin films andare subject to low production levels, batch operations and severe sizelimitations.

The heat decomposition of aluminum compounds is recorded in theliterature. Aluminum hydride has been decomposed into aluminum andhydrogen. Vapor phase plating by decomposition of such aluminumcompounds has been disclosed.

With gasor vapor phase plating, a number of practical problems areencountered. The systems must be tightly sealed against atmosphericcontamination, constant pressure conditions maintained and gas flowrates controlled with narrow limits. Also, many problems exist invaporizing decomposable materials, and in almost all cases a carrier gasmust be provided to give direction to the fiow of the decomposable gas.All additives to the gas stream, such as modifiers, must also bevolatile at the operating temperature and the rates of addition care-3,464,844 Patented Sept. 2, 1969 sive process equipment (see Berger, US.Patent 3,041,-

Berger, in the US. patent supra, makes a major break through in aluminumplating by use of solution plating. In his system a heat decomposablealuminum compound in a liquid or solid solution or dispersion, saidliquid or solid being stable and having a boiling point above thedecomposition temperature of the aluminum compound, is put into contactwith a substrate heated to a temperature above the decompositiontemperature causing formation of aluminum on the .substrate. Othermaterials can be present, for example reducers or oxidizers. Thesubstrate can be of any desirable material so long as said substrate canbe heated to a temperature above the decomposition temperature of thealuminum compound. The deposited aluminum film can be anodized byconventional procedures.

In the process of Williams and Kostas in copending application filedMar. 31, 1965, and having Ser. No. 444,- 431, it is disclosed that theBerger process has certain disadvantages such as excessive smoking,splattering and decomposition of the aluminum compound in the solvent.In this copending application it is shown that much superior coatingsare obtained if the solvent for the aluminum compound has a boilingpoint below the decomposition temperature of the aluminum compound.

It is frequently desired to plate an oxidizable substrate. Suchsubstrates are first cleaned of surface oxidation material and areheated to a temperature above the decomposition temperature of thealuminum compound and subsequently coated by the solution. It isnecessary to heat the substrate in a dry box which has been flushed witha inert gas. However, these extremely hot surfaces are very active andare readily oxidized by even trace amounts of oxygen.

It is, therefore, an object of this invention to provide a method ofprotecting a substrate to be plated at high temperatures againstoxidation at said high temperatures. Other objects and advantages ofthis invention will be apparent from the specification and the claims.

These and other objects of the invention are obtained by coating asubstrate prior to heating With a high boiling liquid hydrocarbon.

The surface to be coated should be prefectly clean and free of anyoxides. This cleaning can be mechanical such as by abrasion with steelwool, sandblasting, sandpaper, and the like. The cleaning can also be bychemical means such as ammonia solution of citric acid, dilutehydrochloric acid, dilute phosphoric acid and the like. The chemicalcleaning agent can be washed off with water and dried with suitabledrying agents such as acetone or methanol.

The solvents suitable in the practice of this invention are fullydisclosed by Berger and the copending application supra. They includealkanes, aromatics and paraflins, ethers and substituted compounds ofthis type. These include n-dodecane, 1,2,3,4 tetramethylbenzene,tetralin, naphthalene, diphenyl, anthracene, parafiin distillate,toluene, ethyl benzene, xylene, kerosene and the like. However,hydrocarbons are generally preferred. Suitable bath temperature willdepend upon the solvent used and the aluminum compound. The temperatureshould be below the boiling point of the solvent and also below thedecomposition temperature of the aluminum compound. In general, thesolution temperature will be between 50 C. and about C. We prefer asolvent that has a boiling temperature below that of the decompositiontemperature of the aluminum compound and prefer to maintain the bath ata temperature between 50 and 150 C.

Any aluminum-containing organic compound, capable of decomposing underheat to liberate aluminum and which can be dissolved or suspended in asolvent medium, can be utilized to product an aluminum film on asubstrate in the practice of this invention. As a practical matter, dueto cost, availability, safety, and the temperature to which theSubstrate may be heated, the aluminum alkyls are generally preferred.Examples of such aluminum compounds include aluminum trimethyl, aluminumtri ethyl, tripropyl aluminum, aluminum tributyl, aluminum tri-isobutyl,aluminum tri-isopentyl, aluminum tri-isooctyl and aluminum tridodecyl.Also operable are substituted alkyls, preferably wherein the substituentis a saturated aliphatic radical. Substituents tending to react with thenewly deposited aluminum coating would ordinarily be avoided. Alsomonoand dialkyl aluminums can be used. Examples include ethyl aluminumdihydride and diethylaluminum hydride, di-isopropylaluminum hydride,aluminum dibutyl hydride, aluminum di-isobutyl hydride and the like. Ingeneral, the preferred material is a trialkylaluminum or adialkylaluminum hydride wherein each alkyl radical contains one to fourcarbon atoms, and, in particular, we prefer aluminum triethyl anddiethyl aluminum hydride.

The aluminum alkyl in solvent can vary from to 90 weight percent, andmost generally will be present in the range 30 to 70 weight percent.

As has been indicated, the use of various minor components in solventmedium can be employed to cause modification of the physical andchemical properties of the film deposited. Such materials as wettingagents to promote adhesion, oxidizing and reducing compounds can beemployed. The precoating of the substrate by the practice of thisinvention protects the substrate from such compounds and permits it tomodify only the coating.

The substrate treatment coating material of thi invention is a heavyhydrocarbon having a boiling point in excess of the decompositiontemperature of the aluminum alkyl. They include the well known mineraloils and waxes, diphenyl alkanes which contain two phenyl groupsdistributed randomly along a normal paraffin, preferably of a C to Cbackbone and dialkylbenzenes, preferably with each alkyl containing 10to 14 carbon atoms and mainly meta and para isomers prepared byalkylation of benzene with chloroalkanes. These materials are obtainedfrom petroleu m, generally by fractionation in various temperature rangecuts and have no fixed chemical makeup. Such compounds generally have aboiling range from 250 to 350C. It would be expected, therefore, thatthe coating material would be completely evaporated under thetemperature conditions of the furnace (400-550" C.). At these":temperatures one would expect to find decomposition of the mineral oiland fouling of the substrate; however, it has been found that nodecomposition occurs, and the substrate is substantially free of oxides.The resulting aluminum plate i far superior to plate prepared underidentical conditions except for the coating. Examples of suitablecoating materials then include, but are not limited to, mineral oils,paraflin waxes, diphenyl decane, diphenyl undecane, diphenyl dodecane,diphenyl tridecane, diphenyl tetradecane, didecyl benzene, diundecylbenzene, didodecyl benzene, ditridecyl benzene and ditetradecylbenzene.

The aluminum plate produced by the process of this invention isflexible, the coated material, e.g. iron strip, can be bent and twistedrepeatedly without breakingthe coating-The coating is more resistant totape-pull-test than are other coatings. This test comprises pressingtape on the coating and pulling it off. Bright, shiny, lustrous aluminumcoatings are obtained.

The substrates to be treated by the method of this invention are onesthat can be oxidized. In general this includes metal substrates andparticularly metal strips. Most frequently, the metal to be treated willbe in iron alloy such as mild steel. However, other metals such ascopper strips or articles can be advantageously treated by the method ofthis invention.

The plating can be done by any suitable method. The plating bath(aluminum compound in solvent) will be maintained at a temperature belowthe boiling point of the solvent and below the decomposition temperatureof the aluminum compound and generally above ambient temperature,however, the initial bath temperature can be at room temperature. Thebath temperature will be determined largely by the nature of thesolvent, the aluminum compound and the temperature differential of bathand substrate desired. As a general rule a temperature in the range 50to C. will be employed; however, any temperature up to the boiling pointof the solvent can be employed.

The substrate, after cleaning and removal of oxides will be coated withthe heavy hydrocarbon and then heated to a temperature above thedecomposition temperature of the aluminum compound. This temperaturewill, of course, depend upon the particular aluminum compound and willbe sufficiently high to offset the cooling effect of the bath. Thetemperature ordinarily employed ranges from about 400 to 550 C. When thesubstrate is heated to much below 400 C., the coating is tough andresist flexing, however, such coatings are unusually thin. Severaltreatments can improve the thickness. When the temperature exceeds about550 C., the coatings tend to flake or crack on flexing. Where theselimitations are not important, we can use temperatures from about 300'to 1000 C.

The article to be plated is then contacted by the plating bath while atthis high heat. The contacting can be done by dipping the article in thebath, passing it through the bath, spraying with the bath or any othersuitable means. Since frequently these solvents are flammable, it ispreferred that the bath be in an oxygen-free atmosphere. It is alsowithin the scope of the invention to heat the substrate in theoxygen-free atmosphere. However, with the coating treatment of thisinvention, the operation can be performed in the natural atmosphere andin small amounts of oxygen in the oxygen-free atmosphere can betolerated. That is, the system would not require the taking of extensiveprecautions to exclude all oxygen. However, even in oxygen-freeatmosphere, more uniform and stronger coatings have been obtained thanwhen the coating was not applied. When the coating material hassutficiently high boiling point, the coating material can be held at thedesired temperature and the article heated in the coating bath and thencontacted with the plating bath.

To further illustrate the invention, the following examples are given.It is to be understood that these serve to illustrate the invention andare not to be considered limiting.

EXAMPLE I Several runs were made wherein thin strips of mild steel wereplated. The plating operation was carried out in a nitrogen purged drybox. The heating furnace was also in the dry box.

A 50% solution of diethylalminum hydride in a hydrocarbon wash oil washeated in an oil bath to about 150 C. Two clean mild steel coupons (1 x3 x 0.03 inch) were coated with a thin film of mineral oil. They werethen placed in a muffle furnace and heated to 500 C. The coupons werethen removed from the furnace and dipped into the plating bath, i.e. thealkyl aluminum solution.

The plated coupons were repeatedly bent and twisted with no adverseeffect.

EXAMPLE II The above run was repeated except the two coupons were coatedby dipping into a 20% solution of mineral oil in toluene.

Again the strips showed no adverse effect from twisting and bending.

EXAMPLE III This run was the same as run 1 except the coupons were notfirst coated.

The plating was uniform but failed when bent or twisted.

EXAMPLE IV Oven Conditions Flex Time Temp., 0. Tape pull test test Run:

1 1min... 500 Good Good. 2 1min 500 Having thus described the invention,what is claimed 1. A process for plating aluminum on a metal substratewhich comprises preparing a solution of an aluminum capable ofdecomposing under heat in an organic solvent having a boiling pointbelow the decomposition temperature of the aluminum alkyl, coating saidsubstrate with a heavy hydrocarbon having a boiling point above thedecomposition temperature of said aluminum alkyl, heating the coatedsubstrate to a temperature above the boiling point of said heavyhydrocarbon and contacting the heated substrate with said solution.

2. The process of claim 1 wherein said heavy hydrocarbon is a mineraloil.

3. The process of claim 2 wherein the aluminum alkyl is diethylaluminumhydride.

4. The process of claim 2 wherein the aluminum alkyl istriethylaluminum.

5. The process of claim 2 wherein the aluminum alkyl istri-isobutylaluminum.

6. The process of claim 2 wherein the aluminum alkyl isdi-isobutylaluminum hydride.

7. The process of claim 2 wherein the aluminum alkyl istriethylaluminum.

8. The process of claim 2 wherein the aluminum alkyl isdi-n-butylaluminum hydride.

9. A process for plating aluminum on a metal substrate comprisingpreparing a solution of an aluminum alkyl in a hydrocarbon solventhaving a boiling point below the decomposition temperature of saidaluminum alkyl, said aluminum alkyl being present in the range of 10 toweight percent; coating said substrate with a film of a heavyhydrocarbon having a boiling point above the decomposition temperatureof said aluminum alkyl, heating the coated substrate to a temperatureabove the decomposition temperature of the aluminum alkyl and above theboiling point of said heavy hydrocarbon; and thereafter contacting thehot substrate with said solution.

10. The process of claim 9 wherein the substrate is heated to atemperature in the range 300-1000 C.

11. The process of claim 4 wherein the substrate is heated to atemperature within the range 450550 C.

12. The process of claim 10 wherein the aluminum alkyl isdiethylaluminum hydride.

13. The process of claim 12 wherein said substrate is contacted byspraying.

14. The process of claim 10 wherein the aluminum alkyl istriethylaluminum.

15. The process of claim 10 wherein the aluminum alkyl istri-isobutylaluminum.

16. The process of claim 10 wherein the aluminum alkyl isdi-isobutylaluminum hydride.

17. The process of claim 10 wherein the aluminum alkyl istripropylaluminum.

18. The process of claim 10 wherein the aluminum alkyl isdi-n-butylaluminum hydride.

References Cited UNITED STATES PATENTS 2,523,461 9/1950 Young et a1.11747 2,619,433 12/1952 Davis et al 117-107.2 X 2,698,811 1/1955 Legg11746 3,041,197 6/1962 Berger 11747 3,075,858 1/1963 Breining 1171073,155,532 11/1964 Basile 117 47 3,198,167 8/1965 Bakish et a1. 117-l07.2X 3,305,386 2/1967 Hafiver et al. 117107.2 X

ALFRED L. LEAVITT, Primary Examiner A. GOLIAN, Assistant Examiner US.Cl. X.R. 1l7-50, 105,

iii/ 1f UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent 3,464,544 Dated September 2, 1969 Inventofls) Billv J. Williams It iscertified that error appears in the above-identified patent and thatsaid Letters Patent are hereby corrected as shown below:

Column 1, line +3, "disclosed" should be --disclose-.-. Column 2, line55, "a" should be --an--. Column 3, line 4, "product" should be--produce--. Column 5, line 13, before "alkyls" insert --aluminuJn--.Claim 1, line 3, before "capable" insert --alkyl--. Claim 7, line 2,"triethylaluminum" should be --tripropylaluminum--.

SIGNED AND SEALED MAY 5 1970 (SEAL) Attest:

d d M. Fletcher, 11-

mum: E. *saaumm, m. Attefl g Comisaioner o1 Patents

